1. Field of the Invention
The present invention relates to a bidirectional optical amplifier circuit for optically amplifying optical signals transmitted in both directions in a bidirectional optical amplifier repeater or a bidirectional regenerative repeater, or for optically amplifying optical signals transmitted in one or both directions in a bidirectional optical end office.
2. Description of the Related Art
In fiber-optic communication systems using optical amplifiers which directly amplify optical signals, output powers from the optical amplifiers can reach 100 mW or more. In such systems, if the output end of an optical amplifier is exposed for some reason such as a removed connector on the output end, there is a danger that a human eye may accidentally be exposed to the high output light, causing damage to the eye. Also, reflected light at the exposed end may be fed back into the optical amplifier, causing oscillations. It is therefore imperative to provide a protection circuit that can automatically lower the output power of the optical amplifier by detecting the exposure of the output end thereof by some means.
In Japanese Patent Unexamined Publication Nos. 4-324335 and 5-83201, there is disclosed a circuit for detecting the exposure of an output end of an optical amplifier by extracting and detecting light reflected from the exposed end of the output fiber by using an optical coupler provided on the output side of the optical amplifier.
Generally, reflected light is weak; for example, in the case of an optical fiber whose end is PC-polished, light attenuated by about 14 dB due to Fresnel reflection is reflected back into the fiber when the connector is removed. When the connector is fitted, reflections at connected ends still occur, and light attenuated by about 22 dB is reflected back. When detecting reflections, discrimination must be made between these two kinds of reflections.
However, as long as the configuration is employed in which reflected light is extracted for detection on the output side of the optical amplifier, it is difficult to achieve reliable detection of reflected light at low cost, for reasons hereinafter explained.
Reflection detecting means, consisting of an optical coupler and a photodiode, are often integrated into optical circuitry on the output side of the optical amplifier. This increases the chance of stray light entering the detecting means. When detecting reflected light, since the light to be detected is weak, even the slightest amount of stray light can contribute noise, degrading the S/N ratio. Further, in a configuration employing backward pumping in which pumping light is injected from the output side of the optical amplifier, the possibility of stray light further increases since pumping light having a far greater power passes in the vicinity of the reflection detecting means.
Besides stray light, there are other factors that can contribute noise: i) Residual reflections from a collimator at the exit end take the same path as the reflected light from the exposed end and enter the photodiode provided to detect the reflected light. ii) If internal reflections occur in an output light detection photodiode provided to control the gain of the optical amplifier, such reflections pass through the optical coupler and enter the photodiode provided to detect the reflected light. In the present situation, to cope with factor i), parts are strictly selected, and to cope with factor ii), an expensive photodiode having low internal reflection is used. These factors have impeded the reduction of the cost.
Hence, there is a first problem that detection of reflected light is extremely difficult and costly.
For a bidirectional fiber-optic communication system using optical amplifiers, a method has been devised in which a pair of optical amplifiers on the upper and lower transmission lines are pumped using a single pumping laser diode from the standpoint of space, power consumption, cost, etc.; that is, using an optical splitting means such as an optical coupler, the output of the pumping laser diode is split into two approximately equal parts. This method is particularly significant for an optical subscriber system. However, in a transmitter/receiver such as an optical end office, a regenerative repeater, etc., the optical post-amplifier in the transmitting side is required to produce a larger optical power than the optical preamplifier in the receiving side. Hence, there is a second problem that it is difficult to apply this method for such applications.